Why does Split Pea Soup Turn Solid when you put it in the Fridge?

Have you ever wondered why split pea soup has to be made from split peas, rather than whole ones? I know I have. After all, once they’ve been reduced to pottage. who cares whether they were originally intact spheres or, one must presume, had been carefully split in two at the factory by a tribe of elves wielding tiny machetes?

In truth, the split pea is a specific variety of pea whose proper name is field pea, originally native to southwestern Asia and one of the earliest crops cultivated by humans. They have a weak plane or layer that splits apart when the peas are dried. That easy-splitting phenomenon, also found in some minerals, is what mineralogists call cleavage. (Now get your mind back on what I’m talking about!)

All peas are varieties of Pisurn sativum n. The field pea is to be distinguished from the common garden pea, a.k.a. green pea or English pea, which is usually sold fresh and still in its pod, to be shelled by grandmothers seated at the hearth or, when fresh and young (the peas, not the grandmothers), to be eaten whole, pod and all. The French are particularly fond of their tiny young peas, which they have imaginatively named petits pois, or “little peas.”

Your solidified soup phenomenon is not unique to peas; it is caused by the peas’ abundance of starch, and exactly the same thing will happen in any starchy soup or sauce, such as a gravy that has been thickened with cornstarch or flour and subsequently refrigerated. They, too, will be unyieldingly thick and gelatinous after having been refrigerated.

Here’s a simplified version of what’s going on.

Under a low-power microscope, starch looks like a collection of translucent round or ovoid capsules of various sizes, called granules. Inside each granule are millions of invisibly small starch molecules, arranged in a relatively organized pattern.

As we heat a starch-bearing food in water, the starch granules become dispersed throughout the liquid, where they gradually absorb water and swell. The swollen granules then rub up against one another like too many overfed fish in too small a pond, and the entire mixture becomes more viscous, or thicker. Some of the granules may even burst and spill their contents, further thickening the soup or sauce into a sort of paste. This entire process is generally called gelatinization.

A dish containing gelatinized starch is just fine if you eat it while it’s hot. The smooth, silky texture of a properly made flour or starch-thickened white sauce, for example, is one of gastronomy’s great pleasures.

But when the starchy leftovers are cooled in the refrigerator, two processes take place in succession: first, gelation (not gelatinization), and then retrogradation. (Note that gelatinization, becoming gelatin-like, took place upon heating, while gelation, formation of a gel, takes place upon subsequent cooling.)

In the first cooling stage, gelation, starch molecules inside the swollen granules slow down because of the falling temperature. (All molecular motion is slower at lower temperatures.) The starch molecules can then begin to mesh with one another and tangle together, forming a weblike structure that traps a lot of water. That kind of structure, a semi-solid mass containing a large amount of locked-in liquid, is called a gel. As the gel continues to cool and more starchto-starch bonds form and tighten the net, some previously trapped water may even be squeezed out, a phenomenon known as syneresis. The soup or sauce “weeps,” and little beads of water can be seen on the surface.

After several hours of cooling and aging, the starch molecules are so tightly tangled with one another that they are no longer dispersible in water. If you add water to try to thin it out, the thick, gummy mass refuses to break up and return to its earlier consistency, because in their tangled gel form the starch molecules cannot unbond from one another to make room for the water and then swim freely into it. In short, the starch has retrograded, gone back, to an insoluble form. You can then try beating it into submission by adding water and whisking vigorously while heating, but it may never be as smooth as it was just after being cooked.

Don’t despair. I’m told that the traditional Dutch pea soup (erwten-soep), also called snert (that’s right, snert), is deliberately made a day ahead and refrigerated, so that when reheated, it will be thick enough to support a spoon standing straight up. When much of your country is below sea level, I guess you have to do something for fun.